These investigations are concerned largely with the immunopathology of schistosomiasis mansoni which has been shown to be due to soluble antigens secreted by eggs trapped in the host tissues. Three antigens, identified by their reactivity with sera from chronically infected mice, were isolated and purified. One of them (MSA1) a glycoprotein with an MW of 138,000 daltons is completely stage and highly species specific as demonstrated by competitive radioimmunoassay. This technique was also used as a highly sensitive and specific means of immunodiagnosis. The inflammatory cells of the granulomas around S. mansoni eggs were differentiated and quantified. Since inflammation is succeeded by fibrosis, in vitro collagen synthesis was studied using liver slices from mice with schistosomiasis incubated with radioisotope-labeled proline. The eosinophil is prominent in the granuloma reaching a maximum of 71% of the cells in the lesions. Using a monospecific antieosinophil serum (AES) previously developed in our laboratories we have found a circulating eosinophilopoietic factor and isolated it on a Sephadex G-25 column. Since AES only affects mature eosinophils, it has enabled us to obtain large numbers of eosinophil precursors from the bone marrow of treated mice and produce a specific antiserum against them in rabbits. Toxoplasma gondii infection and genetically determined diabetes mellitus, both of which strongly suppress cell-mediated immunity, markedly inhibit the development of hepatosplenic schistosomiasis.